Geology
Pulu Marya
Photo courtesy of Fusion Films
A Cocos (Keeling) Island beach
The Cocos (Keeling) Islands consist of two separate coral atolls, 24 km apart, which have formed atop an old volcanic seamount that rises from a depth of 5,000 m in the north-eastern Indian Ocean. Bathymetric research shows that the islands' foundations are actually two of a series of undersea features known as the Vening Meinesz Seamounts. This range of mountains also takes in Christmas Island and extends in a north-easterly direction from a prominent seafloor feature of the Indian Ocean known as the Ninetyeast Ridge. The Cocos atolls are two peaks in a section of the range known as the Cocos Rise (Jongsma 1976). The two atolls are connected by a narrow submarine bank at a depth of 700-800 metres (Gibson-Hill 1948).
The Cocos (Keeling) Islands were the only coral atolls that Charles Darwin visited in 1836 when he developed his well known theory of atoll formation (Darwin 1842). He considered that the upgrowth of coral reefs continued long after the seamounts that supported them had subsided. While others have presented alternative theories, Darwin's subsidence theory of coral reef development has gained wide acceptance.
North Keeling Island is surrounded by a broken, irregular fringing reef, except at the northwest corner. Like the shingle bank around the shore of the Island, the reef is narrower on the sheltered sides of the island and broader on the exposed sides. On the east coast it is continuous across the mouth of the lagoon, and forms a wide bar which almost blocks the entrance.
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